1. Field of the Invention
The present invention relates to a tunnel magnetoresistive effect element for generating so-called magnetoresistive (MR) effect in which a resistance value changes with application of a magnetic field from the outside, a method of manufacturing a tunnel magnetoresistive effect element and a magnetic memory device fabricated as a memory device capable of storing information by the use of a tunnel magnetoresistive effect element.
2. Description of the Related Art
In recent years, as information communication devices, in particular, personal small information communication devices such as portable terminal devices (e.g. personal digital assistants) are widely spreading, it is requested that devices such as memories and logic devices comprising these information communication devices or portable terminal devices should become higher in performance, such as they should become higher in integration degree, they can operate at higher speed and they can consume lesser electric power. Particularly, technologies that can make nonvolatile memories become higher in density and larger in storage capacity are becoming more and more important as complementary technologies for replacing hard disk devices and optical disk devices with nonvolatile memories because it is essentially difficult to miniaturize hard disk devices and optical disk devices because they have their movable portions (e.g. head seek mechanism and head rotation mechanism).
Flash memories using semiconductors and an FeRAM (ferro electric random access memory) using a ferro dielectric material are widely known as nonvolatile memories. However, flash memories are able to write information at speed in the order of microseconds and encounters with a defect such that they are slow in speed as compared with a DRAM (dynamic random access memory) and a SRAM (static random access memory). Further, it has been pointed out that the FeRAM cannot be rewritten so many times.
A magnetic memory device called an MRAM (magnetic random access memory), that had been described in xe2x80x9cWang et al IEEE Trans. Magn. 33 (1997), 4498xe2x80x9d, receives a remarkable attention as a nonvolatile memory that can overcome these defects. The MRAM is a nonvolatile memory from which information can be read out in a nondestructive fashion and which can be accessed randomly. In addition, the MRAM has the following characteristics.
Specifically, the MRAM is simple in structure and therefore can be easily integrated at high integration degree. Further, since the MRAM is able to record information by rotation of magnetic moment, it can be rewritten a large number of times (e.g. more than 1016 times). Furthermore, it is expected that the MRAM has very high access time and it has already been confirmed that the MRAM can be operated at speed in the order of nanoseconds (e.g. speed lower than 5 nanoseconds). From these characteristics, there is a strong possibility that MRAMs will become a main current in the field of memory devices.
Such MRAM uses a tunnel magnetoresistive effect element as a memory element for recording information. A tunnel magnetoresistive effect element has a trilayer structure composed of ferromagnetic material/insulating material/ferromagnetic material, i.e. ferromagnetic tunnel junction (MTJ (magnetic tunnel junction)) if it is of tunnel magnetoresistive effect (TMR (tunnel magnetoresistive)) type. In the MTJ structure, when the magnetization direction of one ferromagnetic material is used as a fixed layer and the magnetization direction of the other ferromagnetic material is used as a free layer, a resistance value of a tunnel current changes depending upon the magnetization direction of the free layer. To be more in detail, when an external magnetic field is applied to the ferromagnetic material layers under the condition in which a constant current flows through the ferromagnetic material layers, MR effect appears in response to a relative angle of the magnetizations of the two ferromagnetic material layers. When the magnetization directions of the two ferromagnetic material layers are anti-parallel, a resistance value becomes the maximum. When the magnetization directions of the two ferromagnetic material layers are parallel to each other, a resistance value becomes the minimum. Therefore, in response to the magnetization direction of the storage layer, the TMR type tunnel magnetoresistive effect element (hereinafter simply referred to as a xe2x80x9cTMR elementxe2x80x9d) is able to store therein information in the form of xe2x80x9c1xe2x80x9d when magnetization is oriented to a certain direction and is able to store therein information in the form of xe2x80x9c0xe2x80x9d when magnetization is oriented to the other direction. Further, the TMR type tunnel magnetoresistive effect element becomes able to readout the states of these magnetization directions in the form of a difference current under a constant bias voltage or in the form of a difference voltage under a constant bias current through a TMR effect.
A changing ratio xe2x80x9cof a resistance value in the TMR element is expressed asxe2x80x9d =2xc2x7P1xc2x7P2/(1xe2x88x92P1xc2x7P2) where P1, P2 represent spin polarizability of the respective ferromagnetic material layers. A spin polarizability represents a difference between the number of electrons (one unit of very small magnets) that are rotating (spinning) upwardly in the solid material and the number of electrons that are spinning downwardly in the solid material. A magnitude of spin polarizability is specified by compositions of magnetic materials comprising mainly a ferromagnetic material layer. Accordingly, since the changing ratioxe2x80x9d of the resistance value increases as the spin polarizabilities P1, P2 of the respective ferromagnetic material layers increase, if the ferromagnetic material layer is made of a magnetic material having a composition with high spin polarizability, then a TMR ratio (ratio between a resistance value in the high resistance state and a resistance value in the low resistance state) of the tunnel magnetoresistive effect element containing the ferromagnetic material layer can increase. As result, excellent information read characteristics can be realized in the MRAM.
To this end, in most cases, the TMR element uses any one of Fe group ferromagnetic material elements such as Fe (iron), Co (cobalt) and Ni (nickel) that are magnetic materials having compositions with high spin polarizabilities or alloy of a combination of more than two of the above-mentioned Fe group ferromagnetic material elements as a material to form the ferromagnetic material layer. As an insulating material layer sandwiched between these ferromagnetic material layers, there is generally used an Al2O3 (alumina) layer that is obtained after a thin film conductive layer of Al (aluminum), which is a nonmagnetic metal material, for example, had been oxidized by native oxidation in the atmospheric pressure during a long period of time or had been oxidized by plasma oxidation or radical oxidation which are known as xe2x80x9cstrongxe2x80x9d oxidation methodsxe2x80x9d. The reason for this is that, because the insulating material layer functions as a tunnel barrier layer to generate TMR effect, not only the spin polarizability of each ferromagnetic material layer should increase but also the insulating material layer interposed between these ferromagnetic material layers should be made uniform and thin in order to obtain a large TMR ratio.
To realize excellent read characteristics in the MRAM, it is very effective in increasing TMR ratios of respective TMR elements comprising the MRAM and is also effective in suppressing dispersions of resistance values among the TMR elements. Therefore, if dispersions of resistance values among the TMR elements are suppressed while the TMR ratios are being increased, then it becomes possible to realize an MRAM that can operate at higher speed and which can be integrated with higher integration degree.
However, the TMR ratio and the dispersion of the resistance value in the TMR element depends considerably upon the characteristics of the insulating material layer (tunnel barrier layer) comprising the TMR element. Accordingly, depending upon the characteristics of the insulating material layer, there is a possibility that undesirable results such as decrease of a TMR ratio and increase of dispersion of a resistance value will be brought about. In particular, in the above-mentioned TMR element according to the related-art, since the insulating material layer sandwiched between the ferromagnetic material layers is obtained in such a manner that an Al conductive layer is oxidized by a suitable oxidation method such as, native oxidation, plasma oxidation or radical oxidation after one ferromagnetic material layer had been formed and the Al conductive layer had been deposited on this ferromagnetic material layer, there arises a problem that will follow.
FIG. 1A is a diagram showing a schematic arrangement of an Al conductive layer 22 deposited on a ferromagnetic material layer 21. As shown in FIG. 1A, the Al conductive layer 22 has a polycrystalline structure and is composed of a set of crystal grains 22a and grain boundaries 22b. In the MTJ structure, since a resistance value depends upon a thickness of an insulating material layer in an exponential fashion, flatness (homogeneity) of the insulating material layer becomes very important. However, if the Al conductive layer 22, which serves as the base of the insulating material layer, has the polycrystalline structure, then since the crystal grains 22a of various sizes exist in the Al conductive layer 22, there is a risk that an insulating material layer having satisfactory flatness will not be obtained. In addition, when the Al conductive layer 22 having the polycrystalline structure is oxidized, first, oxidation starts along the grain boundaries 22b to oxidize the grain boundaries 22b selectively. After the grain boundaries 22b had been oxidized, oxidation proceeds to the insides of the crystal grains 22a to oxidize the insides of the crystal grains 22a. 
FIG. 1B is a diagram showing the state in which the Al conductive layer 22 on the ferromagnetic material layer 21 is oxidized by native oxidation. As described above, when the Al conductive layer 22 is oxidized, first, oxidation proceeds to the grain boundaries 22b to selectively oxidize the grain boundaries 22b. From this reason, since native oxidation has low activation energy, although portions near the grain boundaries 22b are oxidized as shown in FIG. 1B (see portions shown hatched in FIG. 1B), the crystal grains 22a cannot be oxidized sufficiently up to the insides thereof. There is a risk that a resultant insulating material layer will become a tunnel barrier layer having a small oxygen containing amount and whose effective tunnel barrier height is low. When the height of the tunnel barrier layer is low, the TMR ratio decreases rapidly in accordance with increase of a voltage bias, giving rise to deterioration of an S/N (signal-to-noise ratio) obtained when information is read out from the MRAM. Furthermore, if the oxygen containing amount of the insulating material layer is not constant, there is then a possibility that dispersions of resistance values among respective TMR elements will increase.
On the other hand, FIG. 1C is a diagram showing the state in which the Al conductive layer 22 deposited on the ferromagnetic material layer 21 is oxidized by plasma oxidation or radical oxidation. Because the plasma oxidation or the radical oxidation are known as xe2x80x9cstrongxe2x80x9d oxidation methods, even when oxidation proceeds from the grain boundaries 22b to oxidize the grain boundaries 22b, the crystal grains 22a can be oxidized sufficiently up to the insides thereof (see portions shown hatched in FIG. 1C). On the other hand, during oxidation is proceeding up to the inside of the crystal grains 22a, oxidation that had proceeded up to the grain boundaries 22b reaches to the ferromagnetic material layer 21. There is a risk that an interface portion 21a between the Al conductive layer 22 and the ferromagnetic material layer 21 also will be oxidized as shown in FIG. 1C. If the interface portion 21a between the Al conductive layer 22 and the ferromagnetic material layer 21 is oxidized, then the spin polarizability in the ferromagnetic material layer is lowered, giving rise to decrease of the TMR ratio. With respect to this point, although it is considered that intensity of oxidation and oxidation time may be suppressed so as to prevent the ferromagnetic material layer 21 from being oxidized, in that case, similarly to the case of the native oxidation, there arises a problem that the crystal grains 22a cannot be oxidized sufficiently up to their insides.
In view of the aforesaid aspect, it is an object of the present invention to provide a tunnel magnetoresistive effect element, a manufacturing method thereof and a magnetic memory device in which dispersions of resistance values among respective elements can be suppressed while a large TMR ratio can be obtained.
According to an aspect of the present invention, there is provided a tunnel magnetoresistive effect element having a multilayer film structure including two ferromagnetic material layers and a barrier layer sandwiched between these ferromagnetic material layers, wherein the barrier layer is formed by oxidizing a conductive layer and the conductive layer is formed by adding a material of an element different from a metal material to said metal material serving as a principal component thereof.
According to other aspect of the present invention, there is provided a manufacturing method for manufacturing a tunnel magnetoresistive effect element having a multilayer film structure including two ferromagnetic material layers and a barrier layer sandwiched between these ferromagnetic material layers. This manufacturing method is comprised of the steps of depositing one of said ferromagnetic material, thereafter, depositing a conductive layer formed by adding a material of an element different from a metal material to said metal material serving as a principal component thereof onto said one ferromagnetic material layer, forming the barrier layer by oxidizing the conductive layer, and depositing the other ferromagnetic material layer on the barrier layer in later stage.
In accordance with a further aspect of the present invention, there is provided a magnetic memory device including a tunnel magnetoresistive effect element having a multilayer film structure including two ferromagnetic material layers and a barrier layer sandwiched between these ferromagnetic material layers and which is able to record information by the use of change of magnetization direction of the ferromagnetic material layer, wherein the barrier layer is formed by oxidizing a conductive layer and the conductive layer is formed by adding a material of an element different from a metal material to said metal material serving as a principal component thereof.
According to the tunnel magnetoresistive effect element having the above-described arrangement or the tunnel magnetoresistive effect element manufactured by the manufacturing method having the above-described process or the magnetic memory device having the above-described arrangement, since the conductive layer is formed by adding the material of the different element to the metal material, the material of the different element can suppress growth of crystal grains and the sizes of crystal grains in the conductive layer can be reduced as compared with the case in which the metal material does not contain added material of an element. Therefore, since the crystal grains comprising the conductive layer can be reduced in size, flatness in the conductive layer can be improved. Further, since the ratio at which the crystal grains occupy the conductive layer increases as the crystal grains become smaller in size, there is formed the conductive layer that can be easily oxidized. That is, the crystal grains can be sufficiently oxidized up to their insides by an oxidation method of which the activation energy is small. Furthermore, even in the strong oxidation, it becomes possible to end the oxidation to the insides of the crystal grains in the conductive layer before the ferromagnetic material layer is oxidized by optimizing intensity of oxidation and oxidation time and the like.